This invention relates to printheads preferably used with drop on demand ink jet printers. Present drop on demand printing devices make use of a variety of mechanisms to generate ink droplets. Some devices use remote solenoid valves to feed nozzles through a length of tubing, others use solenoid valves directly operating at the nozzles or solenoids operating valves at the nozzles connected via rigid or flexible cables. These valve systems generally produce larger drops that are suitable for large character printers typically used to print on cardboard boxes, etc.
There are a number of drawbacks encountered with such devices, particularly when remotely located solenoid valves are employed. These include low frequency response due to slow valve action causing long filaments of fluid (ink) to be ejected from the nozzle; the high cost of the valves and assorted control problems due to the ingestion of air into the nozzle, all of which cause drop marking errors. Finally, such systems require relatively large amounts of power to actuate the valve drivers.
Valves that operate directly at the nozzle or by means of a flexible cable or rod have shown poor reliability. In addition, manufacturing costs have been high and low frequency response is also found in such devices.
Another type of drop on demand printer produces relatively small characters, for example, document printers. These devices use the surface tension of the orifice as the valving mechanism. Surface tension at the orifice (nozzle) provides the necessary valving after droplet ejection. It absorbs the recoil of the ink drop momentum to stop the ink from leaking and holds the fluid at the orifice while the ink chamber is being refilled. To accomplish this result, however, the orifice used must be small (usually below 60 microns) and the surface tension of the fluid must be high. This small orifice approach precludes the use of these devices for producing large drops and, consequently, large character printing.
The present invention, although suitable for producing small drops and small characters, has a particular advantage with respect to large character printing. Because high surface tension pressure is not required, the size of the orifices can be larger and therefore capable of producing larger drops. Furthermore, these larger orifices can easily be closely spaced to form a single nozzle; the drops from such individual orifices combine to form an even larger drop.
Other advantages of the present invention over prior art include a relatively high frequency response (greater than 500 drops/sec); and positive, locally generated, pressure pulses rather than a remote pressure source assuring better control and fewer operating problems.
Finally, the present invention is susceptible of a lowcost construction employing a series of plates permitting inexpensive photo-etch fabrications, and diffusion bonding joining.
It is accordingly an object of the present invention to obtain the benefits of these advantages over the known art.